GM Cadillac 1967-1989 Repair Guide

Positive Crankcase Ventilation

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OPERATION



All engines covered in this guide are equipped with a positive crankcase ventilation (PCV) system to control crankcase blow-by vapors. The system functions as follows:

When the engine is running, a small portion of the gases which are formed in the combustion chamber leak by the piston rings and enter the crankcase. Since these gases are under pressure, they tend to escape from the crankcase and enter the atmosphere. If these gases are allowed to remain in the crankcase for any period of time, they contaminate the engine oil and cause sludge to build up in the crankcase. If the gases are allowed to escape into the atmosphere, they pollute the air with unburned hydrocarbons. The job of the crankcase emission control equipment is to recycle these gases back into the engine combustion chamber where they are reburned.

The crankcase (blow-by) gases are recycled in the following way: as the engine is running, clean, filtered air is drawn through the air filter and into the crankcase. As the air passes through the crankcase, it picks up the combustion gases and carries them out of the crankcase, through the oil separator, through the PCV valve, and into the induction system. As they enter the intake manifold, they are drawn into the combustion chamber where they are reburned.

The most critical component in the system is the PCV valve. Located in the valve cover or intake manifold, this valve controls the amount of gases which are recycled into the combustion chamber. At low engine speeds, the valve is partially closed, limiting the flow of the gases into the intake manifold. As engine speed increases, the valve opens to admit greater quantities of the gases into the intake manifold. If the valve should become blocked or plugged, the gases will be prevented from escaping from the crankcase by the normal route. Since these gases are under pressure, they will find their own way out of the crankcase. This alternate route is usually a weak oil seal or gasket in the engine. As the gas escapes by the gasket, it also creates an oil leak. Besides causing oil leaks, a clogged PCV valve also allows these gases to remain in the crankcase for an extended period of time, promoting the formation of sludge in the engine. **see Section** 1 for PCV valve replacement intervals.

SERVICE



See Figures 1, 2, 3, 4, 5 and 6

Slow, unstable idling, frequent stalling, oil leaks, and oil in the air cleaner are all signs that the PCV valve may be clogged or faulty. Follow the PCV valve testing procedure in Routine Maintenance and replace the valve if necessary. Check the valve at every tune-up. Remove the valve by gently pulling it out of the valve cover or manifold, then open the clamp on the hose end with a pair of pliers. Hold the clamp open while sliding it an inch or two down the hose (away from the valve), and then remove the valve. If the end of the hose is hard or cracked where it holds the valve, it may be feasible to cut the end off if there is plenty of extra hose. Otherwise, replace the hose. Replace the grommet in the valve cover if it is cracked or hard, and replace the clamp if it is broken or weak. In replacing the valve, make sure it is fully inserted in the hose, that the clamp is moved over the ridge on the valve so that the valve will not slip out of the hose, and that the valve is fully inserted into the grommet in the valve cover.



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Fig. Fig. 1: Schematic of a common PCV system



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Fig. Fig. 2: Cross-section of a common PCV valve



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Fig. Fig. 3: PCV valve and hose between the rocker cover and throttle body - 1986-89 Eldorado and Seville



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Fig. Fig. 4: V6 PCV valve



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Fig. Fig. 5: View of a related emission hose - note the PCV valve is installed in the valve cover



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Fig. Fig. 6: Removing the PCV valve from the valve cover

 
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